Aluminum metal is conventionally produced by the electrolytic reduction of alumina dissolved in a molten cryolite bath according to Hall-Heroult process.
This process for reducing alumina is carried out in a thermally insulated cell or "pot" which contains the alumina-cryolite bath. The cell floor, typically made of a carbonaceous material, overlies some of the thermal insulation for the cell and serves as a part of the cathode. The cell floor may be made up of a number of carbonaceous blocks bonded together with a carbonaceous cement, or it may be formed using a rammed mixture of finely ground carbonaceous material and pitch. The anode, which usually comprises one or more carbonaceous blocks, is suspended above the cell floor. Resting on the cell floor is a layer or "pad" of molten aluminum which the bath sees as the true cathode. The anode, which projects down into the bath, is normally spaced from the pad at a distance of about 1.5 to 3.0 inches (3.81 to 7.61 centimeters). The alumina-cryolite bath is maintained on top of the pad at a depth of about 6.0 to 12.0 inches (15.24 to 30.48 centimeters).
As the bath is traversed by electric current, alumina is reduced to aluminum at the cathode and carbon is oxidized to its dioxide at the anode. The aluminum thus produced is deposited on the pad and tapped off periodically after it has accumulated.
For the electrolytic process to proceed efficiently, the alumina reduction should occur onto a cathode surface of aluminum and not the bare carbonaceous surface of the cell floor. Therefore, it is considered important for the pad to cover the cell floor completely.
As molten aluminum does not readily wet or spread thinly on carbonaceous materials, the pad can best be visualized as a massive globule on the cell floor. In larger cells, the dense currents of electrolysis give rise to powerful magnetic fields, sometimes causing the pad to be violently stirred and to be piled up in selected areas within the cell. Therefore, the pad must be thick enough so that its movements do not expose the bare surface of the cell floor. Additionally, the anode must be sufficiently spaced from the pad to avoid short circuiting and to minimize reoxidation of aluminum.
Still, the movements of the pad have adverse effects which cannot always be readily controlled. For a given cell operating with a particular current of electrolysis, there is an ideal working distance between the cathode and the anode for which the process will be most energy efficient. However, the required spacing of the anode due to turbulence of the pad prevents this ideal working distance from being constantly maintained. Further, since the pad is in a state of movement, a variable, nonuniform working distance is presented. This varible interelectrode distance can cause uneven wear or consumption of the anode. Pad turbulence can also cause an increase in back reaction or reoxidation at the anode of cathodic products, which lowers cell efficiency. In addition, pad turbulence leads to accelerated bottom liner distortion and degradation through thermal effects and through penetration by the cryolite and its constitutents.
It has been suggested in the literature and prior patents that certain special materials such as refractory hard metals (RHM), most notably titanium diboride (TiB.sub.2) or its homologs, can be used advantageously in forming the cell floor. Further, it has been found that RHM tile materials may be embedded into the cell floor, rising vertically through the molten aluminum layer and into the cryolite-alumina bath, with the uppermost ends of these tiles forming the true cathode. When such a cathode design is employed, precise spacing between the true or active surfaces of the cathode and the anode may be maintained, since such a system is not affected by the ever-moving molten aluminum pad acting as the true cathode surface.
Ideally, in contrast to conventional carbon products, these RHM materials are chemically compatibile with the electrolytic bath at the high temperatures of cell operation and are also comparable chemically with molten aluminum.
Furthermore, the special cell floor materials are wetted by molten aluminum. Accordingly, the usual thick metal pad should no longer be required, and molten aluminum may be maintained on the cell floor as a relatively thin layer and commensurate with amounts accumulating between the normal tapping schedule.
With all their benefits to the reduction process, there are problems associated with the use of RHM tiles as vertically projecting members into the alumina-cryolite bath. When attached to carbonaceous substrates, such as the carbonaceous cathode of a reduction cell, erosion occurs at the RHM tile-carbonaceous substrate interface in the presence of molten aluminum and electrolyte. It is believed that this erosion is primarily chemical in nature, with the molten aluminum wetting the tile surface and reacting with the carbon to form Al.sub.4 C.sub.3, which then dissolves in the electrolyte. This sets up a mechanism for removal of carbon from the tile interface and below, causing detachment of the cathodic tiles from the carbonaceous substrate.
Additionally, RHM tile materials are brittle and subject to breakage during the normal working operations performed on a reduction cell. As an increasing number of tiles are broken, the true cathode again becomes an uneven surface. However, due to the presence of the unbroken tiles, it is impossible to adjust the anode to form an even surface.
Recently, it has been proposed to replace the fixed RHM tiles with RHM pieces, with the pieces forming a packed bed on the carbonaceous cathode and within the aluminum pad. U.S. Pat. Nos. 4,396,481 and 4,410,403 and International Application No. PCT/US/81/00067 describe such a cell construction. These are, however, problems associated with this approach.
First, the packing density of RHM pieces is quite high, being in the order of about 150 to 250 pounds per cubic foot. Thus, for example, a reduction cell having a nominal 52 cubic foot volume aluminum metal pad could require over 10,000 pounds of such pieces. RHM material is quite expensive, thus, the high cost of packing a cell with RHM pieces is difficult to cost justify.
The high density of RHM pieces when in a packed bed also creates operational difficulties. An equal volume of aluminum metal is displaced from the metal pad by the packed bed of RHM material. In the 52 cubic foot aluminum pad example given above, a packed bed of RHM pieces could displace almost 37 cubic feet of aluminum metal, or over 70% of the aluminum volume.
The limited volume available for aluminum metal in the cathode cavity due to the high packing density of RHM pieces could cause rapid, high fluctuations of metal level in the cell, resulting in operational difficulties due to excessive anode adjustment, or shorting between the anode and the aluminum cathode, during metal production and/or metal taping operations. Further, the limited space between the RHM pieces, due to the high packing density of these pieces, results in reduced flowability of aluminum metal between the pieces. A gradual accumulation of sludge or muck in the packed bed results, due to natural occurrence of undissolved alumina, causing an increase in the electrical resistance of the cell and inefficient operation. Additionally, the current distribution in the packed bed becomes poor as a result of the increased resistance due to sludge accumulation. The uneven current distribution increases the metal movement and metal wave amplitude in the aluminum metal pad. As a result, the cell must be operated at a higher electrical energy consumption rate, and at an increased anode-cathode distance, due to the higher cathode resistance and metal pad movement.
It is desirable, therefore, to construct an alumina reduction cell which employes RHM materials, for their beneficial effects on cell production, but which eliminates the problems of both fixed tile breakage and packed bed inefficiencies.